The present invention relates in general to pressure sensors, and, in particular, to an integrated pressure sensor and a related process of fabrication using surface micromachining.
Pressure sensors are used in several applications, particularly in processes of active noise cancellation, and especially in distributed parameter systems, such as an airplane cabin or the interior of a vehicle. Semiconductor pressure sensors are widely used for these purposes. Their sensing element is a vibrating silicon diaphragm topped by a spaced backplate layer provided with a plurality of openings and formed of polycrystalline silicon (polysilicon or briefly xe2x80x9cpolyxe2x80x9d), both disposed over a microphone cavity.
These pressure sensors are constructed starting from a semiconductor structure obtained by defining the area of the pressure sensor and implanting a dopant to form a doped layer of a certain type in a monocrystalline silicon substrate of doping type opposite to that of the doped layer that is finally created as a doped buried layer upon growing an epitaxial layer of the same type of doping of the substrate thereon. The epitaxial layer in the area of the sensor will eventually become the diaphragm of the sensor.
By etching from the substrate, that is etching the substrate from the back side thereof to reach the buried doped layer, the latter is uncovered and thereafter selectively etched to leave a microphone diaphragm provided by the epitaxial layer overhanging a so-realized cavity (microphone cavity). This known technique requires that the mask or masks used to carry out the etchings on the back side of the substrate must be perfectly aligned with the masks that are normally used on the front side of the semiconductor wafer for realizing a suitable backplate structure of the pressure sensor above the diaphragm.
This is an unavoidable drawback because of the special equipment that is needed, the relative poor precision of alignment that can be achieved, and the relative high costs. Moreover, the selective chemical etching of the buried doped silicon layer is typically carried out at a relatively high temperature (in the range of 85-90xc2x0 C.), typically using an aqueous solution of potassium hydroxide KOH, to achieve a sufficiently fast etching rate.
A further drawback of the known process is that the etching rate (microns of material removed per minute) in a direction normal to the plane of the substrate is typically 0.3 xcexcm/min, while the speed of the etching in a direction parallel to the plane of the substrate is about 0.03 xcexcm/min, that is ten times smaller. This produces a typically V-shaped groove 12, as depicted in FIG. 1. The V-shaped groove 12 is formed in the silicon substrate 11 using the masking layer 15 to form the membrane area 13 of the membrane layer 14 over the groove.
Such a V-shaped cross section of the microphone cavity may be observed in known semiconductor microphones of different kind, such as those depicted in FIGS. 2a-2d, whether they are piezoresistive (FIG. 2a), piezoelectric (FIG. 2b), capacitive (FIG. 2c) or optically read (FIG. 2d). This represents a tolerated, but far from optimal, shape because of the non-uniformity of pressure wave reactions over the microphone sensing area.
The rear of the microphone cavity is successively closed by the so called xe2x80x9cwafer bondingxe2x80x9d technique. In other words, the sensor is bonded onto a flat silicon wafer, in which the associated circuitry may be integrated.
A method for realizing integrated CMOS structures by first realizing portions of circuits on a separate wafer and thereafter carrying out a wafer bonding, is described in the U.S. Pat. No. 5,659,195. In practice, the above described known process, which is relatively complicated by the wafer bonding step, does not provide for a monolithically integratable pressure sensor. Moreover, it is not possible to vary the depth of the microphone cavity that is predetermined by the thickness of the substrate.
It has been found and is the object of the invention a process for realizing pressure sensors monolithically integratable together with a semiconductor integrated circuit on the same chip, and which overcomes the above mentioned limits and drawbacks of prior art techniques.
According to the present invention, monolithically integrated pressure sensors of outstanding quality and versatility are produced through micromechanical surface structure definition techniques typical of MEMS (MicroElectroMechanical Systems). More precisely, the object of the invention is a process of fabrication of a pressure sensor that may comprise forming in a monocrystalline silicon substrate of a certain type of conductivity a buried layer of opposite type of conductivity upon growing an epitaxial layer of the same type of conductivity of the substrate. The method may also include forming a sacrificial layer of oxide over the epitaxial layer, forming a polysilicon backplate layer with a plurality of holes above the area of the sensor on the sacrifical oxide layer, and chemically etching the sacrifical oxide layer through the holes of the polysilicon backplate layer thereby removing the sacrificial oxide in the sensor area. In addition, the method may include forming a microphone cavity in the sensor area under the epitaxial layer diaphragm by selectively etching the doped silicon of the buried layer.
Differently from the known processes, according to this invention a microphone cavity in the semiconductor substrate may be monolithically formed by carrying out the following steps before forming the sacrifical oxide layer:
1) cutting by plasma etching the front side or the back side of the silicon wafer to form a plurality of trenches or holes deep enough to extend for at least part of its thickness into the buried layer to be selectively etched;
2) electrochemically etching through such trenches the silicon of the buried layer with an electrolytic solution suitable for selectively etching the doped silicon of the opposite type of conductivity, thereby making the silicon of the buried layer porous; and
3) oxidizing and leaching away the silicon so made porous.
Narrow trenches or holes, reaching into the buried layer are realized in the first step, while in the second step the silicon of the buried layer is rendered highly porous. The so made porous silicon may be then easily oxidized and a final chemical etching of the oxidized silicon may be carried out at significantly lower temperatures than those that are normally required by prior art techniques based on the selective etching of the doped monocrystalline silicon. The trenches or holes for accessing the doped buried layer may even be cut through the epitaxial layer and not through the rear of the monocrystalline silicon substrate. This may avoid the burden of precisely aligning the mask on the rear surface with the masks that are used on the front surface of the substrate. Moreover, the thickness of the substrate is normally greater than that of the epitaxial layer. Thus, the need to cut relatively deep and narrow trenches requiring the use of special plasma etching equipment may be avoided.
Optionally, the porous silicon of the buried layer may be oxidized immediately after having performed the selective electrochemical etching or it may be oxidized later in the process, after having deposited the polysilicon layer of backplate.
A further advantage of the present invention is to provide pressure sensors with a cavity that may be wholly defined in a monolithic semiconducting substrate and whose shape and dimensions may be freely established and with a diaphragm that may be shaped in any desired manner leaving to the designer an unrestrained choice of layout.
A further advantage of the invention is that of allowing the realization of a monolithically integrated system for detecting the direction of the source of a sound wave, and capable of exploiting to this end an array of integrated pressure sensors of the invention disposed according to a certain layout.